A Code Division Multiple Access (CDMA) system has three system time, i.e. base station sending system time, Access Terminal (AT) receiving system time (coincident with AT sending system time), and base station receiving system time. As shown in FIG. 1, 0(n) represents a sequence of continuous zeros, and the time measurements are carried out at the sector antenna and an AT RF connector. There are four erect thick lines in FIG. 1, and from the top to the bottom, the first line represents the base station sending system time, the second line and the third line represent the AT receiving system time, and the fourth line represents the base station receiving system time. The AT receiving system time is delayed relative to the base station sending system time, and the base station receiving system time is also delayed relative to the AT receiving system time.
Suppose that signals of a Base Transceiver Station (BTS) can be propagated rectilinearly to an AT without any consideration of other propagation paths. The base station sending system time is taken from Global Position System (GPS). When the BTS sends a signal A, the time delay for the signal A to arrive the AT is L1/c (wherein, the L1 is the distance from the BTS to the AT, and the c is the velocity of light), and this is the delay reason of the AT receiving system time relative to the base station sending system time. A High Rate Packet Data (HRPD) system regulates that the AT sending system time coincides with the AT receiving system time. The time delay of the base station receiving system time relative to the AT receiving system time is also L1/c.
It can be seen from the above that the time delay of the base station receiving system time relative to the base station sending system time is totally 2×L1/c (In particular, the formula for the multi-aspect soft handover, i.e. multi-legs soft handover, is slightly different, which will be described in detail later). Suppose that the delay of the base station receiving system time relative to the base station sending system time is loop Round Trip Delay (RTD), thus a center of a searching window is created. In other words, a Channel Element (CE) of the BTS needs to defer the base station receiving system time for the RTD relative to the base station sending system time in order to demodulate AT signals correctly. Because the base station sending system time is unified, but the base station receiving system time may change with the positions of the BTS and the AT that communicates with the BTS dynamically, as well as the uncertainty of electric wave propagation in practice, the AT signals can not be necessarily demodulated correctly by the base station receiving system time which is deferred for the RTD. Therefore, the channel element actually searches the AT signals by shaking right-and-left within certain limits with the RTD as the center. When the signals are acquired, they will be locked. The shaking limit from the right to the left is the width of the searching window.
To determine the AT system time at the condition of multi-aspect soft handover: though different BTSs have identical base station sending system time, because the distances from the different BTSs to the same AT are different, the time delays Td of the signal propagation are also different. The protocol C.S0024-A v3.0 of the 3rd Generation Partnership Project 2 (3GPP2) stipulates that the smallest Td should be adopted by the AT. In other words, when the multiple BTSs send the same signal A, the BTS that makes the signal A arrive at the AT first (the distance between the BTS and the AT is the shortest) will be adopted by the AT to determine its own system time. The corresponding pilot of BTS is the one that arrives at the AT earlier in the two BTSs in FIG. 2. Hereinafter, suppose that the AT determines its own time by the pilot of the BTS 1.
It can be seen from FIG. 2 that the delay time of the AT system time relative to the sending system time of the BTS 1 is L1/c. The receiving system time of the BTS 1 is 2×L1/c, i.e. the RTD of the BTS 1 is 2×L1/c. While the receiving system time of BIS 2 is the sum of the time delay L1/c of the AT system time and the time delay L2/c that is from the AT to the BTS 2, i.e. the RTD of pilot of the BTS 2 equals to L1/c+L2/c.
A Pseudo-Noise (PN) offset is actually the phase of 15-bit short code pseudo-random sequence, the unit of which is 64 CHIP. The PNs of all BTSs in the system are the integral multiples of a parameter called PILOT_INC in the system. For instance, if the PILOT_INC4, then the PN=0, 4, 8, 12, 16 . . . . When the AT finds a new pilot, a Route Update (RU) message is reported, one field of which corresponds to the new pilot: PN_PHASE, the unit of which is CHIP. The meaning of the field is: a phase of the new pilot measured by the AT according to the current AT system time. The PN_PHASE of the new pilot in the RU does not necessarily equal to an integral multiple of PILOT_INC×64. But it is obvious that the PN of the new pilot should be the PN corresponding to the PN_PHASE which is the most close to an integral multiple of PILOT_INC×64. So the PN can be calculated with the formula below, the unit of which is CHIP:
                    PN        =                              [                                          PN_PHASE                +                                  32                  ×                  PILOT_INC                                                            64                ×                PILOT_INC                                      ]                    ×          PILOT_INC                                    (        1        )            In the above formula, all variables take the round-off numbers, and the content within needs to be rounded down.
The calculation of the center of the searching window of the new pilot: as shown in FIG. 2, suppose that the BTS 2 is at Position P, and the distance from point P to the AT is L1, then the pilot phase difference of the BTS 2 and the BTS 1 will be n which is an integral multiple of the PILOT_INC×64 from the view point of the AT. The PN_PHASE of the pilot of the BTS 2 reported by the AT is also an integral multiple of the PILOT_INC×64, i.e. it equals to PN×64 calculated from formula 1. When the BTS 2 is at Position C as shown in FIG. 2, a balance DELTA needs to be added on the basis of n times of the PILOT_INC×64 to be the pilot phase difference of the BTS 2 and the BTS 1 from the view point of the AT. The DELTA is the propagation time of electric wave from Position C to Position P. The value of the DELTA can be calculated with the formula below:
                                                        DELTA              =                            ⁢                                                (                                                            L                      ⁢                                                                                          ⁢                      2                                        -                                          L                      ⁢                                                                                          ⁢                      1                                                        )                                /                c                                                                                        =                            ⁢                                                PN_PHASE                  ⁢                  _C                                -                                  PN_PHASE                  ⁢                  _P                                                                                                        =                            ⁢                                                PN_PHASE                  ⁢                  _C                                -                                  PN                  ×                  64                                                                                        (        2        )            wherein, the PN_PHASE_C and the PN_PHASE_P are the PN_PHASEs within the RU of the BTS 2 at point C and the point P respectively; the PN is the value of the PN calculated according to the formula 1 when the BTS 2 is at the point C.whereas RTD2, which is the center of the searching window of the BTS 2, is:
                              RTD          ⁢                                          ⁢          2                =                                  ⁢                                            L              ⁢                                                          ⁢                              1                /                c                                      +                          L              ⁢                                                          ⁢                              2                /                c                                              =                                                    2                ×                L                ⁢                                                                  ⁢                                  1                  /                  c                                            +                                                (                                                            L                      ⁢                                                                                          ⁢                      2                                        -                                          L                      ⁢                                                                                          ⁢                      1                                                        )                                /                c                                      ⁢                                                  ⁢                                                  =                                          RTD                ⁢                                                                  ⁢                1                            +              DELTA                                                          (        3        )            wherein the RTD1 is the center of the searching window of the pilot of the BTS 1, which can be obtained after the CE of the BTS 1 acquires the AT. Thus the calculation of the center of the searching window of the pilot of the BTS 2 is finished.
When implementing hard handovers between ANs (Access Networks), the AN on is the source side receives the RU messages reported by the AT, and the CE of the AN on the source side acquires the RTDs of all the PNs in the RU according to the received RU. But because the AN on the target side does not receive the RU message, the RTD of the PN of the AN on the target side cannot be obtained. Thus, the CE of the AN on the target side does not determine its own center of the searching window, and which makes it difficult for the CE of the AN on the target side to acquire the reverse pilot channel of the AT when the AT hands over towards the cell of the AN on the target side.